Re: [PATCHv2 2/3] x86/tdx: Fix race between set_memory_encrypted() and load_unaligned_zeropad()

[Tom Lendacky]

On 5/31/23 15:00, Michael Kelley (LINUX) wrote:
> From: Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> Sent: Tuesday, May 30, 2023 6:22 AM
> > Hi,
> >
> > On 5/30/23 5:57 AM, Tom Lendacky wrote:
> > > On 5/29/23 19:57, Kirill A. Shutemov wrote:
> > > > On Fri, May 26, 2023 at 03:10:56PM -0700, Sathyanarayanan Kuppuswamy wrote:
> > > > > On 5/26/23 5:02 AM, Kirill A. Shutemov wrote:
> > > > > > Touching privately mapped GPA that is not properly converted to private
> > > > > > with MapGPA and accepted leads to unrecoverable exit to VMM.
> > > > > >
> > > > > > load_unaligned_zeropad() can touch memory that is not owned by the
> > > > > > caller, but just happened to next after the owned memory.
> > > > >
> > > > > /s/to/to be ?
> > > >
> > > > Yep, my bad.
> > > >
> > > > > > This load_unaligned_zeropad() behaviour makes it important when kernel
> > > > > > asks VMM to convert a GPA from shared to private or back. Kernel must
> > > > > > never have a page mapped into direct mapping (and aliases) as private
> > > > > > when the GPA is already converted to shared or when GPA is not yet
> > > > > > converted to private.
> > > > >
> > > > > I am wondering whether this issue exist in the AMD code?
> > > > >
> > > > > IMO, you can add some info on the window in set_memory_encrypted()
> > > > > where this race exists.
> > > >
> > > > I don't think AMD affected by load_unaligned_zeropad() the same way as
> > > > Intel does. But I'm not sure.
> > > >
> > > > Tom, do you have any comments?
> > >
> > > Right, shouldn't be an issue for SNP.
> >
> > Thanks for confirming.
>
> Tom -- For my education, could you elaborate on why this problem can't
> occur in an SEV-SNP guest?  There's still a window where the direct map
> PTE and the RMP as maintained by the hypervisor are out-of-sync.  If
> load_unaligned_zeropad() does a read using the direct map PTE during
> this out-of-sync window, isn't that going to trap to the hypervisor?  How
> is the scenario is handled from there to provide the zeros to
> load_unaligned_zeropad()?  I need to make sure Hyper-V is doing whatever
> is needed. :-)

Ah, I think I misunderstood this when it was being talked about. The issue 
SNP would have would be between setting the c-bit but before the PVALIDATE 
is issued. Prior to the RMP being updated, referencing the page will 
generate an #NPF and automatically change the RMP over to private (in 
KVM). However, after the guest is resumed, the page will not have been 
validated resulting in a #VC with error code 0x404 being generated, 
causing the guest to terminate itself.

I suppose, when a 0x404 error code is encountered by the #VC handler, it 
could call search_exception_tables() and call ex_handler_zeropad() for the 
EX_TYPE_ZEROPAD type (ex_handler_zeropad is currently static, though).

Thanks,
Tom

> Thanks,
>
> Michael